This invention relates to a novel method and apparatus for feeding a thin ribbon of cast material, e.g., semiconductor material like silicon, among others, onto a take-up reel in an apparatus for casting a crystalline or coarse grain polycrystalline ribbon of the material. More particularly, this invention concerns a take-up reel and a take-up reel starting feed for use in high speed ribbon crystallization of molten material, in which a starting strip is prewound onto the take-up reel sufficiently to frictionally engage the take-up reel and be thereby drawn onto the take-up reel from a supply reel for the starting strip, and wherein the linear velocity of the starting strip across a contact zone is synchronized with the linear velocity of the ribbon of cast, i.e., crystallized, molten material, which leaves a rotating drum on which the casting is initiated, so that the ribbon of cast material, as it passes over the starting strip in the contact zone, will contact the starting strip and pass onto the take-up reel in contact with the starting strip, and without bunching or significant stretching of the cast ribbon.
In the manufacture of a thin elongated ribbon of cast material by continuous casting of a molten material, e.g., a metal or a semiconductor material, as is more fully explained in the co-pending application of the Applicants filed as Ser. No. 147,804, the disclosure of which is hereby incorporated by reference, a molten ribbon of material to be cast is extruded onto a rotating drum, the surface of which is cool, thereby initiating crystallization of the material. The ribbon of cast material which is thereby formed is a crystalline or coarse grain polycrystalline structure. Such a ribbon casting process can be carried out at very high speeds on the order of hundreds of feet per minute. In the case of semiconductor materials, the ribbons are extremely useful in many applications, including the production of photovoltaic solar cells.
However, the crystalline structure of the ribbon makes the ribbon relatively brittle and subject to damage from bending or stretching. In a high speed ribbon crystallization process, it is very desirable to have a means for taking up the ribbon produced by the crystallization process for storage in a continuous fashion without damaging the ribbon. Using a conventional take-up reel with a mechanism for grabbing the leading edge of the ribbon as the leading edge comes in contact with the take-up reel will cause damage to the ribbon by either crimping the ribbon or excessively stretching the ribbon, either of which may cause the ribbon to crack and break, thereby necessitating a further attempt at feeding the ribbon onto the take-up reel. This is so also because of the usually desirably thin dimension of the cast ribbon. Thus even with cast ribbons, of, e.g., metal, which may have a less brittle crystalline structure than, e.g., semiconductor materials, damage to the ribbon can occur with conventional take-up reel feeds. It is also desirable that the rotating drum be continuously in operation so that crystallization will not build up in the vicinity of the orifice described in the above-referenced co-pending patent application, clogging the orifice. Therefore, in order to avoid waste of the ribbon produced by the crystallization process during the changing of take-up reels from a first reel which is filled with the ribbon of cast material to a fresh reel for taking up additional ribbon, it is desirable to be able to quickly change reels and again wind the leading edge of the ribbon onto the new reel with as little time delay as possible.
The problems enumerated above regarding a high speed formation of a ribbon of crystalline or coarse grain polycrystalline material, are not intended to be exhaustive, but rather are among many which tend to impair the effectiveness of previously known methods and apparatus for taking up the ribbon of cast material formed in a high speed crystallization casting process. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that methods and apparatus appearing in the prior art have not been altogether satisfactory.